Known air conditioning apparatuses for vehicles, such as automobiles, include a compressor that compresses a gas refrigerant, a condenser that condenses high-pressure gas refrigerant by performing heat exchange with outside air, an expansion valve that expands high-temperature, high-pressure liquid refrigerant into low-temperature, low-pressure liquid refrigerant, and an evaporator that evaporates the low-temperature, low-pressure liquid refrigerant by performing heat exchange with outside air, and a refrigerating cycle is formed by connecting these constituent parts via refrigerant flow paths.
In vehicle air conditioning apparatuses of this type, in the case of a configuration using an electric compressor in which a compressor is driven by an electric motor, operation control is performed such that the discharge temperature of the compressor is detected and the rotational speed of the electric compressor is reduced to a set value when the discharge temperature is equal to or higher than a certain temperature. In addition, from the viewpoint of overheating protection, temperature detection means is provided in the vicinity of a motor coil of the electric motor, and control is also performed to reduce the rotational speed of the electric compressor or to stop the electric compressor when the motor coil temperature is higher than an allowable temperature.
As described above, in order to detect the discharge temperature of a compressor or the motor coil temperature of an electric motor, a temperature sensor needs to be installed at the corresponding location; however, from the viewpoint of problems related to a complex configuration and space, as well as increased cost due to a higher number of components, methods for estimating these temperatures on the basis of other parameters have been proposed in sensorless configurations.
For example, Japanese Unexamined Patent Application, Publication No. 2006-291878, titled “CONTROL METHOD AND CONTROLLER FOR ELECTRIC COMPRESSOR”, discloses a method of obtaining a motor coil temperature by performing calculation using a discharge pressure of an electric compressor, electric motor rotational speed, and input current from an electric-motor driver.